Fix bug 6833 in bugzilla

Fix bug 6833 in bugzilla

http://tracker.ardour.org/view.php?id=6833

When searching for our keyboard bindings file, ArdourKeyboard::find_bindings_files() was trying to find files with the wrong extension:- "..keys" rather than ".keys" (note the extra period). Fortunately, some later code was masking the problem (so we were eventually finding the file correctly). However we might as well fix the problem here, just in case we ever need to call ArdourKeyboard::find_bindings_files() from somewhere else in the future.